It is standard practice, as for example known from German patent publication Nos. 2,100,192 and 2,510,127 filed Jan. 4, 1971 and Mar. 6, 1975 by A. Godtner and E. Langecker respectively, to form a thermoplastic resin, such as polyethylene, into a thin-wall tube by injecting the resin at a plurality of angularly equispaced locations into the inner end of a tubular extrusion passage of a ring nozzle. As the still plastic mass moves axially outward along the passage the flows merge and mix, to emerge at the downstream end or mouth of the passage as a unitary tube.
In these arrangements considerable attention is devoted to ensuring that the extruded tube is of uniform wall thickness. Thus various mechanical contrivances are incorporated into the nozzle head to adjust the shape of the passage with an aim to achieving such uniform wall thickness. Even in the best of these systems, however, the wall thickness varies somewhat. The too-thick portions therefore do not expand properly when later processed, leading to failure of the to thin portions.
German patent document No. 2,654,001 filed Nov. 27, 1976 by H. Feuerherm describes both a method and an apparatus of the above-described general type aimed mainly at producing a tube whose wall thickness is uniform. This is accomplished by providing a radially deformable ring that forms an outer wall of the nozzle passage. Various mechanical expedients allow this ring to be deformed so as to vary the passage thickness. Empirically the user of the machine makes various adjustments until reasonably uniform wall thickness is achieved.
The problem with this arrangement is that a setting that is empirically derived for one resin mix will normally have to be changed for another. Furthermore the adjustable nozzle is an extremely expensive and complex piece of equipment which is prone to breakdown.